1. Field of the Invention
This invention relates generally to communication devices, and, more particularly, to a communication device that employs a synchronous primitive signal for coordinating synchronous events.
2. Description of the Related Art
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire, bi-directional communication channel. The Plain Old Telephone System (POTS), designed primarily for voice communication, provides an inadequate data transmission rate for many modern applications. To meet the demand for high-speed communications, designers have sought innovative and cost-effective solutions that take advantage of the existing network infrastructure. Several technological advancements have been proposed in the telecommunications industry that make use of the existing network of telephone wires. One of these technologies is the xDSL technology. DSL technology uses the existing network of telephone lines for broadband communications. An ordinary twisted pair equipped with DSL interfaces can transmit videos, television, and high-speed data.
DSL technologies typically leave the POTS service undisturbed. Traditional analog voice band interfaces use the same frequency band, 0-4 Kilohertz (kHz), as telephone service, thereby preventing concurrent voice and data use. A DSL interface, on the other hand, operates at frequencies above the voice channels from 100 kHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable of offering simultaneous channels for voice and data.
DSL systems use digital signal processing (DSP) to increase throughput and signal quality through common copper telephone wire. Certain DSL systems provide a downstream data transfer rate from the DSL Point-of-Presence (POP) to the subscriber location at speeds of about 1.5 Megabits per second (MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty times faster than a conventional 28.8 kilobits per second (KBPS) transfer rate.
One popular version of the DSL technology is the Asymmetrical Digital Subscriber Line (ADSL) technology. The ADSL standard is described in ANSI T1.413 Issue 2, entitled, xe2x80x9cInterface Between Networks and Customer Installationxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface,xe2x80x9d the most recent revision of which as of the filing date of this specification is incorporated herein by reference in its entirety.
ADSL modems use two competing modulation schemes: discrete multi-tone (DMT) and carrierless amplitude/phase modulation (CAP). DMT is the standard adopted by the American National Standards Institute. The technology employed by DMT ADSL modems is termed discrete multi-tone. The standard defines 256 discrete tones. Each tone represents a carrier signal that can be modulated with a digital signal for transmitting data. The specific frequency for a given tone is 4.3125 kHz times the tone number. Tones 1-7 are reserved for voice band and guard band (i.e., tone 1 is the voice band and tones 2-7 are guard bands). Data is not transmitted near the voice band to allow for simultaneous voice and data transmission on a single line. The guard band helps isolate the voice band from the ADSL data bands. Typically, a splitter may be used to isolate any voice band signal from the data tones. Tones 8-32 are used to transmit data upstream (i.e., from the user), and tones 33-256 are used to transmit data downstream (i.e., to the user). Alternatively, all the data tones 8-256 may be used for downstream data, and upstream data present on tones 8-32 would be detected using echo cancellation. Because more tones are used for downstream communication than for upstream communication, the transfer is said to be asymmetric.
Through a training procedure, the modems on both sides of the connection sense and analyze which tones are less affected by impairments in the telephone line. Each tone that is accepted is used to carry information. Accordingly, the maximum capacity is set by the quality of the telephone connection. The maximum data rate defined by the ADSL specification, assuming all tones are used, is about 8 MBPS downstream and about 640 KBPS upstream.
In present ADSL implementations, bits are allocated to different carriers according to a xe2x80x9cloadingxe2x80x9d algorithm, such as the Water Filling (WF) algorithm or Equal Energy Distribution (EED) algorithm, for example. The aforementioned loading algorithms utilize the signal-to-noise ratio (SNR) profile of a channel and a desired SNR margin to allocate bits. In general, carriers with higher SNR are able to carry more bits than those with lower SNR values. Typically, increasing the desired margin reduces the number of bits that can be carried by a given carrier. These loading algorithms typically attempt to establish either a maximum throughput or start with a predetermined throughput and distribute the bits required to support that throughput to the least impaired tones. After the modem has been trained, dynamic rate adaptation or bit swapping techniques may be used to change the bit rate in response to improving or degrading line conditions.
Modems typically have a layered architecture. The first layer, referred to as the physical layer (PHY) or level 1 is responsible for bit processing functions, such as error checking, modulating, demodulating, scrambling, etc. Signals or messages provided by the physical layer are often referred to as primitive signals, as they do not require interaction with higher levels for detection. The next layer, often referred to as level 2 is responsible for data framing and management functions. Current ADSL receivers employ a combination of level 1, level 2, and physical link management. Level 2 messages are multiplexed into a level 2 data frame and modulated in the level 1 signal. Physical link management messages are communicated in the layer 2 messages, some of which alter the format of the level 1 signal. One such type of physical link management message is a bit swap message used to adaptively change the number of bits or signal strength associated with a particular tone.
A known bit swapping technique is described in U.S. Pat. No. 5,400,322 and incorporated herein by reference in its entirety. The technique involves sending a bit swap request message to the opposing modem informing the opposing modem of the impending bit swap. The opposing modem sends a reply message including a symbol counter value at which to implement the bit swap. Another known technique, described in U.S. Pat. No. 5,479,447 uses a handshaking procedure that implements the bit swap a predetermined number of symbols after receipt of the acknowledgement message. At least one disadvantage of these techniques is that they both assume proper functioning of the modems"" higher level message processing layers to implement operating parameter changes in the physical layer. Also, the exchange protocol is not entirely robust in that the modem sending the acknowledgement message does not affirmatively know that the other modem has received the acknowledgement. A bit error in the multiplexed message may prevent the acknowledgment from being recognized. Accordingly the acknowledging modem could change its operating parameters undesirably and lose its communication link.
To illustrate the complexity of the exchange necessary to transfer and act upon bit swap messages, consider the following illustration. Bit swap messages include encoded parameters that are buffered. During each superframe (i.e., predetermined number of grouped frames there are a small number of bytes available to encode commands, such as bit swap commands. The buffered commands are transferred incrementally, superframe by superframe, byte by byte. Thus, a bit swap message may span more than one superframe. Error checking codes are added during each superframe. The error checking is also completely independent of the bit swap message, so bit swap messages may be included in more than one RS coding block. The bytes get mixed with all the other bytes, including other overhead messages, framing messages, and user bytes. The bits of all these bytes get mixed up per the bit ordering sequence and assigned to tones. The bytes of the bit swap message may end up on different tones and may share tones with other types of information. Eventually the bits are modulated and transferred to the receiving modem. For the bit swap message to finally get received, the inverses of all the processes described above must be successfully be performed.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a communications system including a first a modem and a second modem. The first modem is adapted to modulate and demodulate data in accordance with a set of operating parameters and transmit a request to change one of the operating parameters. The second modem is coupled to the first modem and adapted to modulate and demodulate data in accordance with the set of operating parameters, receive the request to change one of the operating parameters, and send a primitive synchronization signal to the first modem. The first modem is adapted to implement the change in response to receiving the primitive synchronization signal.
Another aspect of the present invention is seen in a method for communicating messages requiring synchronization. The method includes receiving symbols in a modem; analyzing the symbols to identify a primitive synchronization signal; and modifying a physical layer operating parameter of the modem in response to identifying the primitive synchronization signal.